This invention deals generally with a blender for solid particulate matter, and more specifically with a blender of the type which is a cylindrical vessel with a central lift column or pipe through which a gas is pumped to move material from the bottom to the top of the vessel for mixing.
Particulate blenders which are cylindrical vessels with central lift columns to raise bottom material to the top for mixing are well known in the art. They are available in versions which feed the new material into the vessel at either the bottom or the top of the vessel, and both types use a gas pumped up through the central pipe to lift material from the bottom to the top. The simplest such apparatus performs only the lifting action and therefore depends for the mixing action on imperfect mass flow of solids recirculating within the blender. Perfect mass flow within the blender would result in no blending since by definition no variation of downward velocity exists across horizontal planes in the blender tank. This basic flow concept has been traditionally adhered to in order that no dead (stagnant) spots exist in the blender which otherwise can seriously degrade performance. Thus the designer must adhere to the mass flow principal to avoid dead zones but then add devices which systematically withdraw materials from several elevations to produce mixing.
Considerably more complex apparatus has also been developed to enhance the blending action. Much of this depends upon the addition to the vessel of devices called "downcomers", which are separate columns or pipes which rise from the bottom of the vessel to various heights within the vessel, and thereby transport material directly from several different levels within the vessel to the bottom by gravity, from which the material is lifted to the top of the vessel again along with material which has progressed downward with the bulk of the material in the vessel.
While the downcomer system performs the blending function in a reasonably satisfactory manner, it requires a complex and expensive structure. In order to take material from several levels within the vessel and mix them together, there must be many columns of various heights spread throughout the vessel, or there must be multiple columns with controlled openings throughout their lengths. Moreover, each such column or pipe requires a strong support structure, since it is subjected to the movement and stresses of the stored material.